Differentiating between Nanoparticles and Nanobubbles by Evaluation of the Compressibility and Density of Nanoparticles

There are a growing number of reports in the literature of techniques to produce swarms of long-lived nanosized bubbles. These are of interest both because their stability is unexpected and because of the reported wide and growing range of applications for nanobubbles. These reports demonstrate the presence of nanoparticles but generally lack direct evidence that the particles are indeed nanobubbles. Here, we report two methods that are able to distinguish long-lived nanobubbles from other nanoparticles. First, the mean density of nanoparticles in dispersion is determined. Second, the influence of external pressure on the size of nanoparticles is measured. As the density and compressibility of a gas are very different from that of liquids and solids, these methods can differentiate between nanobubbles and other nanoparticles. The resonant mass measurement was adapted to measure the mean density of nanoparticles and showed that candidate nanoparticles were buoyant but with a density of 0.95 g/cm³. Light scattering was used to examine the influence of an applied external pressure of 10 atm on the diameter of candidate nanoparticles. An insignificant change was observed. These results demonstrate that the candidate nanoparticles investigated here are not nanobubbles and cast doubt on many reports of long-lived nanobubbles in bulk. These methods can be applied widely to distinguish nanobubbles from other nanoparticles.